I solved those problems. I was googling hard, i read dozens of bug tickets and launchpad threads, and though i couldn’t really find anything that exactly matched the problems i had, the intersection of the suggestions always included using pppoeconf, so i gave it a try, ran it, then rebooted and now everything works fine and smooth. I still have no idea what the true nature of the problem was, but it’s fixed so i can’t really care anymore.